1. Field of the Invention
The invention relates to apparatuses for forming an image because of an optical scan of laser light by a polygon mirror on which the laser light is incident and which is rotated by a scanner motor, and more particularly to apparatuses in which a periodic signal reflecting the laser light leaving the polygon mirror in an angularly oscillatory manner, and a periodic signal reflecting the rotation of the scanner motor are each useable as a signal indicative of the rotational speed of the scanner motor.
2. Description of the Related Art
There has been already known apparatuses for formatting an image because of an optical scan of laser light by a polygon mirror on which the laser light is incident and which is rotated by a scanner motor. An example of such apparatuses is a laser printer.
For such apparatuses, a technique of speed control of the scanner motor has been already proposed as a technique of controlling the speed of the scanner motor using two periodic signals different in type, one of which is a frequency-generator signal (hereinafter, referred to as “FG signal”) produced as a result of the detection of the rotation of a rotary body rotating with the scanner motor, the other of which is a beam-detector signal (hereinafter, referred to as “BD signal”) produced depending on the scanning period of the laser light.
In general, such apparatuses are each adapted to include a scanner motor driving a polygon mirror for rotation, and a laser light emitter (e.g., a laser diode) emitting laser light onto the polygon mirror.
Such apparatuses are each also adapted to further include a first periodic-signal generator (e.g., a beam detector BD), a second periodic-signal generator (e.g., a frequency generator FG), and a controller.
The first periodic-signal generator detects the laser light reflected off from the polygon mirror, to thereby generate a first periodic-signal (e.g., the BD signal) depending on the scanning period of the laser light. The second periodic-signal generator detects directly or indirectly the rotation of the scanner motor, to thereby generate a second periodic-signal (e.g., the FG signal) depending on the rotational speed of the polygon mirror.
The controller performs speed control of the scanner motor, based on the first periodic-signal outputted from the first periodic-signal generator and the second periodic-signal outputted from the second periodic-signal generator.
For example, Japanese Publication No. Hei 10-035007 of Japanese Patent Application discloses an example of a conventional technique of controlling the speed of a scanner motor using two kinds of periodic-signals (the BD and the FG signal), as described above.
More specifically, in the example, the speed control is performed based on the FG signal at the start-up of a driving operation of the scanner motor, and is switched to the speed control based on the BD signal. That is to say, in the example, the type of the signal for use in the speed control is changed from the FG signal to the BD signal.
Further, in the example, if the rotational speed of the scanner motor exceeds an allowable range during the speed control using the BD signal, then it is determined that the BD signal has become abnormal, resulting in the change-over to the speed control using the FG signal.